Technologies progress with each passing day. In the lighting technology, the advancement is especially significant. The LED technology has been developing actively in recent years. The emittable light spans visible light, infrared light, and ultraviolet light; the light intensity is also raised substantially. With the emergence of white LEDs, the development is gradually directed to lighting applications nowadays.
An LED is monochromatic light source. On the other hand, the white light (the sunlight) in the nature has a wide-band spectrum. Thereby, a single LED cannot emit light with multiple colors; other materials should be used as auxiliaries. In other words, it is impossible that a single LED emits white light. Instead, a white LED is formed by the monochromatic light composed of three primary colors, namely, red (R), green (G), and blue (B), or by converting the monochromatic light emitted by an LED into white light by using fluorescent powders. Then, the overall spectrum contains the spectra of the three primary colors, which stimulate the light sensing cell in human eyes and thus eliciting the sense of white light.
All of the popular white LEDs adopt a single light-emitting unit to emit light with shorter wavelength, such as blue or ultraviolet light. Then, by using phosphor, a portion or all of the light is converted to the light containing green and red light, which have longer wavelengths in spectrum. The conversion of wavelength of light is named fluorescence. Its principle is that after short-wavelengths photons (blue, violet, and ultraviolet light) are absorbed by electrons in the fluorescent material, the electrons are excited to a high-energy-level and unstable excited state. Afterwards, when the electrons return to the original levels, a part of the energy is dissipated in the form of heat, while the rest is released in the form of photons. Because the energy of the released photons is less than before, their wavelengths will be longer. In addition, during the conversion process, a portion of energy is converted to heat, which is energy loss. Thereby, the efficiency of such kind of white LEDs is lower.
Besides, although the current LED technology is advancing continuously, there are still many drawbacks. High-efficient LED chips are power consuming and elicit high temperature, resulting in waste of power as well as shortening in lifetime. Hence, improvements are required in heat dissipation and color rendering. Traditional bulbs and halogen lamps have superior color rendering; fluorescent tubes also have high color rendering. Illumination by light sources with low color rendering results in an abnormal feel of color. Moreover, it also harms vision and health.
Technical research and development are devoted to reinforcing color rendering. It is generally believed that artificial light sources should enable human eyes to percept colors with correctness, just as seeing objects in the sunlight. Thereby, good color rendering means closer to the real look of objects in the sunlight, while bad color rendering means distortion. If the colors seen in a certain light source are utterly identical to those seen in standard illumination, the general color rendering index (Ra) is defined as 100. Basically, values of the general color rendering index greater than Ra80 will satisfy higher lighting requirements for color visibility. Of course, depending on application scenes and purposes, the requirements can be different. This gauge is the color-rendering property of a light source and named as the general color rendering index. The general color rendering index is the difference between the colors of an object illuminated by a certain light source and a reference light source.
The present invention improves the drawbacks according to the prior art. According to the present invention, a first light-emitting chip and two second light-emitting chips are disposed on a holder for die bonding and wire bonding. The first light-emitting chip emits red light and the plurality of second light-emitting chips emit white light. The red light and the white light are mixed, giving a mixed light with high color rendering and brightness. Objects illuminated by the mixed light will exhibit colors closest to their original colors as perceived by eyes.